1. Field of the Invention
Embodiments of the present invention relate semiconductor integrated circuits and particularly to semiconductor integrated circuits for carrying out stabilization of an output voltage of an insulating transformer.
2. Related Art
A switching power supply system is mounted on an electronic device such as, for example, a cellular phone or a digital camera to be used for stepping up or stepping down an input voltage to be supplied to a load. In such a switching power supply system, states relating to the operations of a power supply are set with various methods according to power supply specifications required by electronic devices.
Important parameters such as a switching frequency with respect to a switching power supply system affect characteristics of the system such as a power supply noise characteristic, dimensions of parts, breakdown voltages of parts and temperature rise of parts, so that there are some ideas of methods of setting such important parameters. In the switching power supply circuit disclosed in JP-A-2007-14082 (see paragraph Nos. [0039] to [0046], etc.), a DC voltage supplied from a DC power supply is stepped up to be supplied to six white light emitting diodes connected in series as a load so as to drive the white light emitting diodes. In the switching power supply circuit, for adjusting the switching frequency, an external resistor as an adjusting resistor is provided for the oscillating circuit of a step up chopper regulator.
When a switching power supply circuit is normally used for electronic equipment such as a portable appliance, a liquid crystal television set or a DVD (Digital Versatile Disc) player, noises generated from the switching power supply circuit may be classified into noises having adverse effects on other integrated circuits in the electronic equipment and noises having no adverse effects. The invention of the switching power supply circuit disclosed in JP-A-2007-14082, since the resistance value of the external resistor and the switching period are in a linear relation to each other, is formed so that the switching period is freely varied by replacing one external resistor to facilitate adjustment of produced noises. However, for applying the invention to a power supply control IC (Integrated Circuit) including a step up chopper regulator, there is left such a problem as to require a specialized terminal for newly connecting a frequency adjusting resistor to the IC.
A power supply control IC for controlling a switching power supply system has the number of terminals come to be limited due to requirements for reductions in cost and in the package area. Therefore, in many cases, power supply control ICs have no specialized terminal provided for setting parameters other than the above explained switching frequency. In such case, for complying with various power supply specifications of electronic equipment, the lineup of power supply control ICs themselves must be made increased.
An increase in the lineup of power supply control ICs by power supply specification of electronic equipment will cause an increase in costs of products. Thus, for reducing the cost of electronic equipment, a product comes to be required which is capable of making selections of a plurality of necessary kinds of operating conditions with one power supply control IC.
As a system addressing such requirement, in JP-A-2007-73954 (see paragraph Nos. [0003] to [0028], etc.), a system is disclosed which system carries out the setting of an operating state during the initialization period of a power supply control IC without increasing the number of terminals of the power supply control IC. The described system, for carrying out selection of a required functional parameter and a required operation mode from a plurality of functional parameters and operation modes, is to actualize settings of a plurality of operating states with a single component by, for example, selecting a specified capacitance value of capacitance values of a multifunctional capacitor coupled to a pin of an integrated circuit. Here, a multifunctional capacitor is a capacitor which has, in addition to functions of setting in the initialization period functional parameters, operation modes or other device characteristics of the integrated circuit to which the multifunctional capacitor is connected, normal functions of some kinds for the normal operation of the integrated circuit. For example, a capacitor such as a power supply pin decoupling capacitor or a feedback pin loop compensating capacitor can be used as a parameter/mode selection capacitor during initialization.
In the system disclosed in JP-A-2007-73954, with the capacitance value of the above explained capacitor such as a power supply pin decoupling capacitor or a feedback pin loop compensating capacitor, a capacitor originally arranged for other purpose, determined for being applied to mode setting, the mode setting is appropriately carried out. However, after the mode setting in the initialization period is ended, the capacitor comes to be used for its original purpose. Therefore, each of the capacitance values of the capacitors such as the power supply pin decoupling capacitor and a feedback pin loop compensating capacitor must be set to a magnitude suited for actualizing its original purpose.
Here, the feedback pin loop compensating capacitor has its original purpose to compensate the frequency characteristic of the feedback pin loop of a power supply control IC and is to be disposed so that no feedback loop for a power supply control in a switching power supply system becomes unstable. The feedback pin loop compensating capacitor, with its capacitance value set to be less than the value optimum for the stability of the feedback loop, causes the system to be unstable to oscillate. Conversely, the capacitance value set to be more than the optimum value causes the response as a power supply system to become slow, which makes the output voltage impossible to recover soon to the variation of a load, for example. Therefore, the capacitance value of the capacitor significantly affects the response of a feedback system to be a problem of restricting the design itself of a switching power supply in that the change in the capacitance value limits the adjustment of phase compensation and causes an abnormal oscillation of the power supply.
Moreover, in general, the following three kinds of cases are considered in the method of supplying a power supply voltage to a power supply control IC. A first case is the case in which like in the case disclosed in JP-A-2007-73954, a power supply voltage is supplied from an external power supply connected to different terminals of the power supply control IC to be produced as a power supply voltage by an internal regulator. A second case is the case in which in an initialization period at startup, a power supply voltage is produced by a startup current produced by a startup circuit (corresponding to the internal regulator in the first case) connected to an external power supply and, in a normal operation, a power supply voltage is supplied from the auxiliary winding of an insulating transformer. A third case is the case in which an external power supply is used.
For using a power supply pin decoupling capacitor as a multifunctional capacitor, a regulator producing a constant current for charging the capacitor is necessary in the power supply control IC. This is because the power supply pin decoupling capacitor is a capacitor to be connected to a power supply pin (between a power supply pin and a ground pin) of the power supply control IC for supplying a power supply voltage to the power supply control IC itself. The power supply pin decoupling capacitor, however, is a capacitor that functions as a buffer when power consumption of the power supply control IC becomes high to cause the supply of power supply voltage to become insufficient or to cause the power supply voltage to vary. In addition, the power supply pin decoupling capacitor also functions for removing noises such as ripples in a voltage on an electric power supply line. Accordingly, for achieving such original object, the capacitance value of the power supply pin decoupling capacitor must be made large to some extent. Thus, when a power supply pin decoupling capacitor with a large capacitance value is to be charged with a constant current in a short time, the value of the constant current supplied from the regulator must be set large, which requires the use of a large-scale regulator for generating a large constant current. Therefore, the use of a multifunctional capacitor causes a problem with respect to cost.
In the third case, a power supply voltage is supplied to the power supply control IC with power supply pins connected to an external power supply, so that the power supply pin decoupling capacitor is brought into a state of being always connected to the external power supply. However, although the power supply pin decoupling capacitor is charged by a constant current in such a state of connection, no distinction is possible as to whether the power supply pin decoupling capacitor is charged by the current from the external power supply or charged by the constant current produced in the power supply control IC. Namely, for using the power supply pin decoupling capacitor for mode setting in an initialization period, the power supply pins must be disconnected from the external power supply and a circuit for the disconnection is to be provided out of the power supply control IC. Thus, the method disclosed in JP-A-2007-73954 can be said to be inadequate for being applied to the third case to set parameter/mode.
As was explained in the foregoing, the method of using the multifunctional capacitor disclosed in JP-A-2007-73954 directly affects the normal function of the power supply control IC. Thus, there were such problems that the usage was difficult, a large scale regulator must be provided to result in an increase in cost, and some power supply voltage supply systems were impossible to be used.
Furthermore, in JP-A-2007-258294 (see paragraph Nos. [0023] to [0027], etc.), there is a description of a semiconductor integrated circuit which is provided so that a mode switching signal is made inputted without increasing the number of terminals. Here, of terminals necessary for a normal operation, a terminal is used to which a voltage is inputted whose voltage range in a normal operation has significant differences between a power supply voltage and between a ground voltage. Thus, a voltage near the power supply voltage or near the ground voltage is inputted to the terminal so that a mode switching of the semiconductor integrated circuit is carried out. Therefore, according to the technique disclosed in JP-A-2007-258294, a semiconductor integrated circuit can be provided to which a mode switching signal can be inputted without increasing the number of terminals.
However, in the semiconductor integrated circuit disclosed in JP-A-2007-258294, feedback terminals, to which the feedback signal of a power supply control IC is inputted, are used so as to carry out switching between a test mode and a normal operation mode. Therefore, for supplying a mode setting signal and a feedback signal to the feedback terminals with the signals switched, a circuit, producing the mode setting signal and, along with this, carrying out switching between the mode setting signal and the feedback signal, must be provided externally to the power supply control IC. The way of mode switching like this, although it causes no problem in a test carried out before mounting the power supply control IC on a product, is difficult to be carried out after the power supply control IC is mounted on the switching power supply system.
For solving above problems in the circuits disclosed in JP-A-2007-14082 and JP-A-2007-258294 and in the system disclosed in JP-A-2007-73954, the applicant submitted international patent application no. PCT/JP2010/054218. The circuit disclosed in the application is a circuit in which a resistor for making a decision with respect to a state is added to a terminal for outputting a control signal or a terminal for inputting a sensing current signal. At the rising of a power supply voltage, the resistance value is read in so as to set an operation mode according to the magnitude of the resistance value. The circuit, however, is operated on condition that the mode setting is carried out in the early stage, so that the circuit does not permit a mode change during operation.
Compared with this, the circuit disclosed in JP-A-2002-136124 is a circuit in which the output of a control winding (auxiliary winding) for detecting the output voltage of a secondary output winding of a transformer is made to be pulled down by a photocoupler to thereby give instructions for a mode change to a control unit. To a power supply control IC forming the control unit, this is to require a specialized terminal receiving the output of the control winding. This is because when forming a switching power supply system which uses a transformer with its input voltage provided by carrying out switching of a DC voltage, to which an AC voltage supplied from an AC power supply is rectified, or of a DC voltage supplied from a high voltage DC power supply, an auxiliary winding is generally used first for supplying a power supply voltage to a power supply control IC forming a control unit.
Namely, when receiving a power supply voltage from the auxiliary winding, for making the supply of the power supply voltage stable, a capacitor with a capacitance being large to some extent is connected to the terminal receiving the supply. Therefore, even though a voltage is supplied to the terminal with the way disclosed in JP-A-2002-136124 applied, a signal for switching the mode sometimes can not be transmitted well. Even in the case when the signal for switching the mode can be transmitted well, the electric charges in the power supply voltage stabilizing capacitor are made discharged to cause problems of making the operation of the power supply control IC become unstable and increasing power consumption. Therefore, an additional terminal is absolutely necessary for being provided for receiving a mode changing signal. A specific example of a configuration provided with the additional terminal will be shown in the following.
FIG. 4 is a circuit diagram showing an example of a configuration of a circuit of a related switching power supply system and FIG. 5 is a waveform diagram showing operation waveforms of basic parts of the related switching power supply system.
The switching power supply system shown in FIG. 4 is a switching power supply carrying out switching using self-excited oscillation and having a transformer T, a semiconductor switch Q1, a control IC 100 and a feedback circuit 11. The transformer T has a primary winding N1, a secondary winding N2 and an auxiliary winding N3. To one end of the primary winding N1 of the transformer T, a DC power supply 12 is connected. The other end of the primary winding N1 of the transformer T is grounded through the semiconductor switch Q1 and a resistor R1 for current detection. The secondary winding N2 of the transformer T is connected to output terminals 13 through a rectifying and smoothing circuit with a diode D1 and an output capacitor C1.
The semiconductor switch Q1 is to be an N channel MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) here. The drain of the semiconductor switch Q1 is connected to the primary winding N1 of the transformer T, the source is connected to the resistor R1 and the gate is connected to a control and output terminal OUT of the control IC 100.
One end of the auxiliary winding N3 of the transformer T is connected to a zero current detecting terminal ZCD of the control IC 100 and to a power supply terminal VCC of the control IC 100 through a diode D2. The other end of the auxiliary winding N3 is grounded.
To the output terminal 13 on the positive polarity side of the switching power supply system, the feedback circuit 11 is connected. The feedback circuit 11 has resistors R2 and R3 both detecting variation of the output voltage at the output terminals 13, a shunt regulator 14, a photocoupler PC1 including a light-emitting side photocoupler element PC1e and a photo-detecting side photocoupler element PC1r, and a resistor R4 for limiting a current. The photo-detecting side photocoupler element PC1r is connected to a feedback terminal FB in the control IC 100 with the collector and is grounded with the emitter. The shunt regulator 14 let a current, corresponding to the variation in an output voltage detected by the resistors R2 and R3, flow in the light-emitting side photocoupler element PC1e and, by the photo-detecting side photocoupler element PC1r, an error signal corresponding to the variation is inputted to the feedback terminal FB in the control IC 100.
Here, when the control IC 100 makes the semiconductor switch Q1 turned-off, no current flows in the resistor R1, by which the level of the signal inputted to a current detecting terminal IS is made to be zero. When a signal making the semiconductor switch Q1 turned-on is outputted from a control output terminal OUT, the semiconductor switch Q1 is made turned-on. This makes a current flow in the primary winding N1 of the transformer T and the semiconductor switch Q1 to make electromagnetic energy stored in the primary winding N1. The electromagnetic energy stored in the period is, during the period in which the semiconductor switch Q1 is made turned-off, released from the secondary winding N2 to the output capacitor C1 through the diode D1.
The output voltage of the switching power supply system is easily affected by variations in the input voltage from the DC power supply 12 and in load. Thus, the feedback circuit 11 carries out feedback of a voltage corresponding to an error to the specified output voltage to the control IC 100 as a feedback signal. By comparing the feedback signal with a voltage signal to which a current flowing in the primary winding N1 and the semiconductor switch Q1 is converted by the resistor R1, a duty as a period during which the semiconductor switch Q1 is turned-on is controlled, by which an operation for stabilizing the output voltage is carried out.
Therefore, as shown in the waveform diagram of FIG. 5, the timing at which the state of the semiconductor switch Q1 is changed from a turned-on state to a turned-off state becomes the time at which a (feedback terminal voltage)/n (n: a certain value), a voltage to which a feedback terminal voltage at the feedback terminal FB is reduced to 1/n (or a voltage to which the feedback terminal voltage is level shifted to 1/n), and a voltage at the current detecting terminal IS becomes equal to each other. While, the timing at which the semiconductor switch Q1 is turned-on is the time at which the detected voltage at the zero current detecting terminal ZCD comes closer to zero. Namely, when the semiconductor switch Q1 is turned-off to allow electromagnetic energy stored in the primary winding N1 to be released to the output side through the secondary winding N2, the voltage across the secondary winding N2 and the voltage across the primary winding N1 are decreased. This state is also transmitted to the auxiliary winding N3. Thus, when the voltage across the auxiliary winding N3 is made inputted to the zero current detecting terminal ZCD to detect that the detected voltage comes closer to zero, the semiconductor switch Q1 is made turned-on. Namely, the zero current detecting terminal ZCD is a terminal for detecting the timing at which the semiconductor switch Q1 is to be turned-on. By repeating turning-on and -off of the semiconductor switch Q1, stabilizing control of the output voltage is carried out.
For example, when the output voltage of the switching power supply system is about to lower, the voltage at the feedback terminal FB comes to increase. Then, the turned-on period of the semiconductor switch Q1 until the feedback terminal voltage/n coincides with the voltage at the current detecting terminal IS (or a voltage to which the feedback terminal voltage/n is level-shifted) becomes longer to increase an amount of the energy stored in the primary winding N1, which acts so as to increase the output voltage.
Moreover, the switching power supply system is provided with a mode switching circuit 15. The mode switching circuit 15 has a mode switching signal input terminal 16, which is connected to the base of a transistor Q2 through a resistor R5. Between the base and emitter of the transistor Q2, a resistor R6 is connected and the emitter of the transistor Q2 is grounded. The collector of the transistor Q2 is connected to the output terminal 13 on the positive polarity side of the switching power supply system through a light-emitting side photocoupler element PC2e of a photocoupler PC2, including the light-emitting side photocoupler element PC2e and a photo-detecting side photocoupler element PC2r, and a resistor R7. The collector of the photo-detecting side photocoupler element PC2r is connected to a switching signal detecting terminal MC and is further connected to the power supply terminal VCC of the control IC 100 through a pull-up resistor R8. The emitter of the photo-detecting side photocoupler element PC2r is grounded.
With a mode switching signal inputted to the mode switching signal input terminal 16 being at an H (High) level, the transistor Q2 is made turned-on to bring the light-emitting side photocoupler element PC2e to emit light. This makes the photo-detecting side photocoupler element PC2r turned-on to bring the electric potential of the switching signal detecting terminal MC to an L (Low) level. Moreover, with the mode switching signal being at an L level, the transistor Q2 is made turned-off to cause the light-emitting side photocoupler element PC2e to emit no light. This turns-off the photo-detecting side photocoupler element PC2r to bring the electric potential of the switching signal detecting terminal MC to become the H level. The control IC 100 carries out switching of a mode depending on whether the electric potential of the switching signal detecting terminal MC is the L level or the H level.
As is explained above, in the circuit of the related switching power supply system, when the switching of a mode is desired, the switching signal detecting terminal MC was independently provided in the control IC 100. Moreover, when the insulation condition between the primary side and the secondary side of the transformer T is desired to be kept, the photocoupler PC2 including the light-emitting side photocoupler element PC2e and the photo-detecting side photocoupler element PC2r were used. Moreover, when switching was carried out between operation modes, a mode switching signal was externally inputted to change the logic state at the switching signal detecting terminal MC and the change was detected by the inner circuit of the control IC 100, by which switching to a specified operation was carried out.
With such way of detecting the switching signal, however, for making the control IC detect an externally inputted switching signal, a specialized terminal must be provided in the control IC. For example, a current standard control IC for a switching power supply for AC to DC conversion or DC to DC conversion is provided with eight pins (terminals). The minimum kinds of necessary functions are incorporated so as to be exhibited through the eight pins. Therefore, when no free pins are provided for new functions, some of present convenient functions must be eliminated for providing the new functions or, in the worst case, the package must be changed for increasing the number of pins. This causes the control IC to be insufficient in functions or to be provided at increased cost.
The invention was made in view of such points with an object of providing a semiconductor integrated circuit that can detect an externally inputted mode switching signal without newly providing any specialized terminal, and a switching power supply system provided with such a semiconductor integrated circuit.